1. Field of the Invention
The present invention relates to systems and methods for using microalgae. In particular, the present invention relates to the use of microalgae for removing pollutants discharged to a body of water from a power plant.
2. Discussion of Background
Water pollution is a major international problem. Polluted waters may be unfit for drinking, irrigation of croplands and recreational uses. Severe pollution may result in permanent destruction of the natural ecosystems in bodies of water and the wildlife that inhabit them.
Annually, in the Unites States alone, many billions of dollars are spent to avoid the pollution of bodies of water. Nonetheless, in some cases, technology does not exist for preventing some forms of pollution from reaching nearby water bodies, leaving only in situ methods for reducing the level of water pollution to acceptable levels.
The waste heat given off by power plants is a particular pollution problem. If discharged directly into a body of water, the waste heat can do substantial damage. Thermal shock can kill fish. Unrestrained algae growth, encouraged by the warm, nutrient-laden discharge waters, can result in eutophication of the cooling body and noxious blooms. The principle alternative to direct discharge to a body of water, cooling towers for discharging the waste heat to the atmosphere, is expensive.
Microalgae are used extensively in the treatment of waste waters because they can sequester large amounts of nutrients and other pollutants. At present, there are many municipal and industrial waste treatment plants that use microalgae in "oxidation" ponds to biodegrade wastes. However, there is no control over the algal species and, with few exceptions, no harvesting of the algal biomass generated in these ponds.
Aquacultural production of microalgae (algaculture) is a rapidly advancing branch of biotechnology. Microalgae are being produced and harvested commercially in several locations in the United States and other countries for the production of health supplements and a variety of other specialty, high-value products. Furthermore, there is now a commerical use of microalgal-based ion exchange systems for the recovery of selected heavy metals and microalgae are a potential source of a very large variety of other high value products such as specialty chemicals, animal feeds, fuels, lubricants, antibiotics and enzyme inhibitors.
Several commercial microalgae production systems exist in the United States. They use 10 to 25 acres of ponds with the one acre "high rate pond" as the basic production unit. In other countries, some algacultural efforts use shallow, diked ponds with little if any management. The major problems in algaculture are species control and harvesting. Species control is currently only achieved when the natural physicochemical conditions of these ponds favors the strains of algae grown therein for harvest.
Harvesting microalgae is difficult because of the small size of the algae. When microstrainers are used, they typically have small screen openings. Screen opening size must be very small to trap the microscopic algae. Such small screen openings can become plugged and impede the flow of water through the screens.
There is a need for a cost effective algaculture process and system for removing pollutants discharged to bodies of water from power plants.